Vector control of AC motors requires feedback from phase currents of a motor. Usually, these currents are obtained by measuring at least two of the phase currents. The currents have to be measured by devices that are electrically isolated from control electronics. Hall-effect sensors, which are commonly used for this purpose, are expensive components in low-cost frequency converters. In addition, deviations in the gains between current sensors of different phases may cause current ripple and, consequently, torque ripple. A cost-effective alternative to the phase current measurement is to measure the DC-link current of a frequency converter as in [1] T. C. Green and B. W. Williams, “Derivation of motor line-current waveforms from the DC-link current of an inverter,” Proc. Inst. Elect. Eng. B, vol. 136, no. 4, pp. 196-204, July 1989. The phase currents of the motor can be estimated using the DC-link current and information on the states of inverter switches.
Previously, several methods have been proposed for the estimation of phase currents. The phase currents can be sampled during the active voltage vectors of a direct torque controlled drive as disclosed in [2] T. Habetler and D. M. Divan, “Control strategies for direct torque control using discrete pulse modulation,” IEEE Trans. Ind. Applicat., vol. 27, no. 5, pp. 893-901, September/October 1991.
A dynamic model of a permanent magnet synchronous motor (PMSM) is used to predict the stator current, and the phase currents are updated from the available current samples during a single three-phase pulse width modulation (PWM) cycle in [3] J. F. Moynihan, S. Bolognani, R. C. Kavanagh, M. G. Egan, and J. M. D. Murphy, “Single sensor current control of ac servodrives using digital signal processors,” in Proc. EPE'93, vol. 4, Brighton, UK, September 1993, pp. 415-421. Three-phase PWM can be used and multiple samples can be taken in one switching period to obtain the phase currents as disclosed in [4] F. Blaabjerg, J. K. Pedersen, U. Jaeger, and P. Thoegersen, “Single current sensor technique in the DC link of three-phase PWM-VS inverters: a review and a novel solution,” IEEE Trans. Ind. Applicat., vol. 33, no. 5, pp. 1241-1253, September/October 1997.
Stator current information is obtained using a model for active and reactive power balance in [5] S. N. Vukosavic and A. M. Stankovic, “Sensorless induction motor drive with a single DC-link current sensor and instantaneous active and reactive power feedback,” IEEE Trans. Ind. Electron., vol. 48, no. 1, pp. 195-204, February 2001. A motion-sensorless scheme using a fundamental-excitation method with space-vector PWM at high speeds and an INFORM method with discrete active voltage vectors at low speeds is proposed in [6] U.-H. Rieder, M. Schroedl, and A. Ebner, “Sensorless control of an external rotor PMSM in the whole speed range including standstill using DC-link measurements only,” in Proc. IEEE PESC'04, vol. 2, Aachen, Germany, June 2004, pp. 1280-1285. Phase currents can also be sampled during the active voltage vectors applied in an additional excitation voltage sequence as disclosed [7] in H. Kim and T. M. Jahns, “Phase current reconstruction for AC motor drives using a DC link single current sensor and measurement voltage vectors,” IEEE Trans. Pow. Electron., vol. 21, no. 5, pp. 1413-1419, September 2006.
Some of the previous methods require modification of the inverter switching pattern [2], [7], which results in voltage and current distortion and additional losses. Methods proposed in [3], [4], [6] employ three-phase voltage modulation, which requires a variable current sampling interval to detect the phase currents. Compared with a fixed sampling interval, faster A/D conversion and signal processor are needed. The benefit of rejecting the switching frequency and its subharmonics by the synchronized sampling is also lost with a variable sampling interval, and additional compensation algorithms have to be applied. The method proposed in [4] requires four current samples in each modulation period to reject the current ripple caused by the inverter.
Vector control of PMSM requires information on the stator currents of the machine. Costs relating to the current measurement are reduced when the current is measured from the DC-intermediate circuit of the inverter and state information on the inverter output switches are used to allocate the measured sample to the correct output phase. The stator current can then be reconstructed from these samples for control purposes.
If the PMSM is to be controlled in low rotational speeds, a signal injection method needs to be used. In signal injection, the machine is fed with a high frequency voltage signal. This injected voltage signal causes a current which can be detected and demodulated for correcting the position estimate of the rotor. One of the problems with the above is that the traditional signal injection does not necessarily give sufficient results in connection with DC-link current measurements. This is due to the fact that DC-current measurement gives samples from the output phase currents rather irregularly, which leads to a situation where the high-frequency current originating from the injected voltage cannot be reliably determined.